1. Field of the Invention
This invention relates to directional couplers and, in particular, to a method and a directional coupler having a switch that provides selective coupling among the signal ports of the coupler.
2. Description of the Prior Art
Television signals are generated by power amplifiers and then combined in high frequency devices for delivery to a broadcast antenna. Should the output signal of any one power amplifier fail, it is extremely important that the remaining signal be delivered to the broadcast antenna with the highest available power.
When a directional coupler is used to combine television signals with amplitudes A and B, the amplitude of the output signal is A+B. For the typical case, A and B are substantially equal and the output signal amplitude is 2A. Should one of the signals fail, e.g., due to power amplifier failure, the remaining signal is coupled to the output port with an amplitude of A/2. To enhance the output signal amplitude to A, a switchless combiner has been used. A switchless combiner utilizes a directional coupler, a phase shifter and a magic-T connected in series. The switchless combiner is costly.
U.S. Pat. No. 4,119,931 discloses a parallel transmission line directional coupler with a switch that has a coupled and an uncoupled position. When in the uncoupled position, the switch isolates the parallel lines, and when in the coupled position, the switch electromagnetically couples the parallel lines.
An object of the present invention is to provide a switchable directional coupler that enhances output signal power without any need for a phase shifter or a magic-T.
A directional coupler according to the present invention has an electrical signal coupling enclosure with first, second, third and fourth ports. A switch has a first position that partitions the enclosure so that an output signal at the fourth port has an amplitude of A+B when input signals with amplitudes A and B are applied to the first and second ports. The switch has a second position that partitions the enclosure so that the output signal has an amplitude of A when only the first input signal is applied to the first port. The switch has a third position that partitions the enclosure so that the output signal has an amplitude of B when only the second input signal is applied to the second port.
The switch includes one or more shutters that are located to establish the first, second and third positions within the signal coupling enclosure. The shutters are slidably inserted and removed via channels disposed in the coupling enclosure.